US 2838420 A
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Description (OCR text may contain errors)
June 10,1958 J. E. VALENTE 2,838,420
. METHOD FOR DRYING IMPREGNATED POROUS WEBS Original Filed May 17, 1952 METHOD FOR DRYING IMPREGNATED POROUS WEBS John E. Valeute, Neenah, Wis, assignor to Kimberly- Clark Corporation, Neenah, Wis., a corporation of Delaware 288,406, May 17,
Continuation of application Serial No.
1956, Serial No.
1952. This application August 23, 605,896
4 Claims. (Cl. 1179-103) This invention relates to the manufacture of impregnated, porous cellulose fiber sheet or web materials, and is particularly concerned with the control of the distribution of the impregnant throughout the thickness of the web.-
The problems dealt with by this invention may arise in connection with the manufacture of a variety of porous cellulose fiberwebs such as water-laid, carded or airlaid webs, etc., in which an impregnant is introduced into the web by means of a liquid vehicle, and subsequently dried to fix or immobilize the impregnant.
.In' the drying of these webs by the use of conventional procedures, it is extremely difiicult to obtain a finished product in which the impregnant is uniformly distributed throughout the thickness of the web because conventional drying methods tend to alter the distribution of the impregnant.
For example, in the manufacture of resin-impregnated creped cellulosic Wadding of a type suitable for fabrication into various products such as filter units and the like,
i. e. arelatively thick, low-density porous web, the web is uniformly impregnated with a thermo-setting resin in a relatively uncured state in the form of a solution or dispersion in water or in a suitable organic solvent. When such impregnated web's are dried in the conventional manner, i. e., by directing streams of heated air against one or both surfaces of the impregnated web, or by contact heating of the web, the uniform distribution of the resin is altered, the resin concentration being rather dense toward one or both surfaces and correspondingly deficient toward the center of the web. While the exact nature and'operations of the forces causing this redistribution is not entirely understood, it is assumed to be the effect of non-uniform drying. In such case, the. resin solution may migrate by capillary action to the relatively drier zone from which evaporation occurs at the greatest rate, with resultant accumulation of the resin solids in that zone.
This condition is undesirable, particularly in the (case of'multi-ply Webs, because a 'deficiency of resin in the intermediate plies mayweaken the inter-ply bond of the ultimate product. For. this reason, it has been necessary in the past to use larger quantities of resin than would be necessary if it were possible to obtain uniform distribution of the resin in the dried, impregnated Web.
Accordingly, it is the principal object of the present invention to provide a method of drying an impregnated porous web in such manner that the distribution of the impregnating material will be uniform throughout the web. A more specific object of the invention is the provision of a method of drying a multi-ply web of creped cellulosic wadding which has been impregnated with a resin solution or dispersion, so as to obtain uniform distribution of the resin throughout the severalconstituent plies. Y In general, the'method of the invention comprises passing a stream of air or gas through a web of porous material containing an impregnant distributed throughout theweb in a liquid vehicle, and reversing the direction in the impregnant throughout the thickness of the web can be. of the web and of the,
attained. According to the nature impregnating material, fixed i. e., incapable of moisture content of the web is still relatively high, as will presently appear. a While the method of the invention may find application in several fields, it was the latter may be effectively and is here described in that connection.
To illustrate, the basiocreped Wadding sheet may have a dryer'basisweight of from 4.5 to 10 lbs. per 3000 square feet, according to its ultimate use, and a crepe ratio of from 1.5 to 3.0. Such sheets have relatively low strength," particularly when wetted with the impregnating solution,
and for greater ease of handling, are generally assembled in from 5 to 10 plies to form a multi-ply web having a dryer basis weight of from 22.5 lbs. to lbs. per 3000 square feet.
The multi-ply web is impregnated with a solution of synthetic resin so as to obtain in the web a residual resin solids content of from 20 to 70 percent, based on the dry Weight of the impregnated web, the usual desired resin content being within the number of available synthetic resins which are water soluble or dispersible are especially suitable, a particular phenolic resin known commercially as R-468, manufactured by the Monsanto Chemical Company, being illustrative. Depending upon the impregnating technique, the desired residual resin solids content, the desired drying load, and other factors, the concentration of resin solids impregnating solution may vary from 30 to 70 percent. The impregnated webs are usually dried to a volatile content of approximately 8 to 11 percent of the finished weight of the Web. In the ultimate use of the impregnated sheet, the resin is cured by heating, and taking a typical filter product as illustrative, the impregnated web may have a specific gravity of from 0.15 to 0.45 in its final state.
Referring to the drawing, the single figure of which illustrates schematically one form of apparatus suitable for carrying out the method of the invention, the dry, unimpregnated web is unwound from a supply roll 5, and passes into a suitable impregnating mechanism 7 which may include an endless belt or screen 9 for transporting the web through a bath 11 of the impregnating solution. The wet impregnated sheet is then carried be-i tween a pair of endless Wire screens 13 through an elongated dryer 15 which is partitioned longitudinally into a series of chambers 17. In the dryer,'air or other'gas, preferably heated, is forced through the web, the direction of the gas flow being reversed in successive chambers. Each of the web-supporting screens 13 is supported and tensioned by a pair of large rolls 19, one at the entrance and one at the exit of the dryer, and the return runs of the screens 13 are supported by guide rolls 21 outside the dryer. On the exit side of the dryer, the dried impregnated web is rewound into a roll 23 for storage and shipment. Itzwill be understood that the screens, the
' supply andre-winding rolls, and the impregnating mecha- Paiented June 10, 1958 web in opposite di,
the temperature of further redistribution, while the developed in connection with, and has particular advantageswhen used in, the manu-. facture of resin-impregnated creped cellulosic Wadding,
range of 30 to 65 percent. A
nisrn, are drivenat correlatedspeeds which maybe varied to control the time required. for a given length of web to traverse the dryer.
The drying medium, preferably hot, relatively dry air is supplied under pressure by a motor driven pump or blower 25 and delivered to the. dryer through a duct system 27 which includes a suitable heat exchanger 29 using steam for example as the heating medium. For purposes of control and heat economy, a portion of the air may be recirculated, a certain amount of fresh air being continuously added and spent air being continuously bled from the system, as at 31 and 33 respectively. To suit varying requirements, it is desirable that the drying medium be available at a variable temperature and, furthermore,. that suitable throttling valves or dampers 35 be disposed in the inlet conduit of each of thechambers for controlling the rate of flow of the drying medium into the individual chambers.
It has been discovered that by forcing the air through the web and by reversing the direction of the air flow under carefully controlled conditions, the migration of resin which accompanies the use of the conventional drying procedures is effectively checked. Theoretically, it may be possible, by forcing the drying medium through the web at a sufiiciently high rate, to achieve a uniform rate of evaporation throughout the web thickness,tbut to the extent that the theoretical is not attainable; the effect of unequal evaporation is overcome by reversing the direction of the flow of the drying medium through the web. In this connection, it has been observed that'the tendency of the resin solution to migrate under the: force of unidirectional heating or drying disappears while the web is still quite wet, and well beiore the-final degree of dryness necessary for satisfactory handling is reached.
When a porous cellulose fiberweb is impregnated with a resin solution having a resin solids concentration of from 30% to 70% by weight, in sufficient quantity to provide in the web a resin solids content of from 20 to 70% by weight of the dried impregnated web, a' portion of the solution not absorbed by the fibers exists as a-continuous liquid body extending throughout the thickness of the web. This relatively free portion of the impregnating solution is present in the interstitial spaces in the web and as a film on the interconnected fibers, and is movable by capillary action, in response to unbalanced evaporation, until it is immobilizedby evaporation of a suificient quantity of the solvent. This immobilization may be partially due to increasing viscosity ofthe free portion of the solution, but examination of the dried web also shows discrete deposits of resin as menisci at the intersections of fibers which indicates that the continuous liquid body hecomes interrupted, breaking down into many smaller bodies between which no transfer, or at best, only limited transfer of the solution is possible.
Thus, whereas an impregnatedweb will exhibit marked migration effects when dried solely by conventional means, or even by forcing air at practical rates through the web in one direction only, an identical web can be dried to an intermediate state of dryness by the reversing, through-drying method of the invention and then carried to final dryness by conventional means without showing any migration effects, even though the web be very soggy when the reversing through-drying is halted.
Specifically, in the case of a 6- ly web of creped cellulosic wadding having a dryer basis weight of 5.9 lbs. per. 3000 square feet and a crepe ratio of about 1.9 and impregnated with an aqueous solution of a water soluble phenol-aldehyde rcsin'containing 33% resin solids so as to achieve a residual resin: solids content of about 46% by weight of the dried impregnated web, air at room temperature was forced through the web in alternately opposite directions at a velocity of about 265 feet per minute until the volatile content of the web was'reduced to about 32% of the total weight of the web. The web was then dried to a final volatile content of 8;?percentby directing a stream of air at a temperature of 176 F. through the web in one direction only. No appreciable migration of resin was found to have occurred, as evidenced by the fact that the resin contents of the individual constituent plies varied by only 1.5% resin solids.
On the other hand, when a comparable specimen of the impregnated wadding was dried from start to finish by directing a stream of air at 225 F. through the web in one direction only at a velocity of 392 feet per minute, the resin solids content of the constituent plies was found to vary from 47.8% to 36.4%, with the higher resin content occurring in the ply on the side ofthe web from which the air entered, and being graded downwardly to the lower figure in the ply on the side of the web from which the air left the web.
From the above observation, it is apparentthat there is an intermediate state of dryness at which the resin solution becomes fixed in the web and incapable of migration, and that the alternation of the How of the drying medium through the web, to be efiective, must occur before and until this critical range or point in the drying process is reached. For cellulose fiber Webs impregnated with aqueous solutions of a phenol-aldehyde thermosetting resin at resin solids concentrations suitable for impregnation, found'to be within the range of from 20% tiles based on the total weight of the web.
For a given type and weight of web impregnated with a specified amount of a solution of known concentration, the critical condition of dryness will occur more rapidly as the'drying rate is increased. Therefore, it is desirable for control purposes that one or several of the factors which determine the drying rate be variable. These include the speed of the web through the dryer, the temperature and degree of saturation of the drying medium, which in the case of air may vary from room condition to relatively dry air at 300 F., and the velocity of the drying medium through the Web. These factors are also related to the length of each of the successive individual dryer chambers or sections which may be made variable to controlthe drying time in each chamber. With a relatively high drying rate, the timing of the alternation of the flow of drying medium is, of course, more critical than when the drying rate is reduced, and in such case, it will be necessary to exercise definite control to make certain that at least one and preferably more reversals offlow of the drying medium occur before the impregnating solution becomes effectively fixed in situ.
To illustrate from actual experience, and referring again to the previously cited specific example of the 6-ply to 30% volaweb which was impregnated with an aqueous solution of a concentration of 33% resin solids, and partially dried to a volatile content of 32% of the total weight of the web by the reverse throughdrying method above described, the dryer was partitioned into eight consecutive chambers, each 24 inches in length, through which the web was moved at a speed of 20fect per minute. As
a phenol-aldehyde resin at described, relatively dry air at room temperature wasforced through the web in opposite directions in successive chambers at a velocity of about 265 feet per second. The partially dried web issuing from the dryer was then completely dried by hot air at 176 F. directed through the web in one direction only, and the resin content of each of the six constituent plies was found to be substantially the same.
On the other hand, when air was dryer at a temperature within the rangeof from 220 F. to 290 F., other conditions remaining approximately content the same, in order to reduce the final volatile to the desired point for convenient handling, that is, approximately 10%, the resin was found to have migrated as severely as though the web had been dried by conventional'surface dryingmeans, and it was necessary to foreshorten the first chamber to a length of about 8 inches inorder to eliminate the migration efiect and to produce this critical degree of dryness has been delivered to the a web in which the resin was uniformly distributed. It is apparent, therefore, that at higher drying rates, the critical state of dryness of the-web is reached very rapidly, and that due care must be taken, as in the instance cited, to assure that the direction of air flow through the web is reversed at least once before the critical state of dryness is reached. By the use of the drying method above described, substantially uniform distribution of the impregnant can be maintained, with the particularly beneficial result in the field of resinimpregnated multi-ply creped cellulosic wadding that sufiicient resin is retained in the intermediate plies to assure good ply adhesion at lower total resin contents than are feasible when conventional drying methods are used. This results not only in substantial savings of resin but also in increased web flexibility, since the excess of resin in the outer plies is substantially eliminated. Moreover, the drying of the webs is accomplished in a fraction of the time, and with equipment which occupies far less space, than that required by conventional surface drying methods.
While the invention was conceived and developed in connection with a particular product, it may be applicable by those skilledin the art to other fields wherein porous webs of various types are impregnated with solutions, dispersions, suspensions or the like, and subsequently dried to deposit the dissolved or dispersed material, and wherein uniformity of distribution of the deposited material is desired. Accordingly, the invention is not to be construed as comprehending only the specific example set forth above, but rather as commensurate with the appended claims.
1. The method of producing an impregnated web in which the impregnant is substantially uniformly distributed throughout the thickness of the web which comprises introducing into a porous web a volatile liquid vehicle in such quantity and containing an'impregnant in low enough concentration that said vehicle is present in said web as a continuous liquid body throughout the thickness of the web and said impregnant is mobile within said web in response to forces established by drying said web, passing a gaseous drying medium through said web in alternately opposite directions before and until said continuous liquid body is interrupted but said web is still Wet with said vehicle and contains a substantial quantity thereof capable of being evaporated by a conventional drying process, thereby to immobilize the impregnant in the web, and then further drying the web sutficiently to permit handling.
2. The method of producing a substantially uniformly dense fiber base plastic sheet which comprises impregnating a porous fibrous web with a resin solution in such quantity and of such resin solids concentration as to be mobile within said web in response to forces established by drying said web and to provide in the web a residual resin solids content of from 20% to by Weight of the dried impregnated web, passing a gaseous drying medium through said web in alternately opposite directions at least until sufiicient solvent has been evaporated to reduce the volatile content of the web to from 20 to 30 percent of the total weight of the web, and then further drying the web sufficiently to permit handling.
3. The method of producing a substantially uniformly dense cellulose fiber base plastic sheet which comprises impregnating a low-density, porous cellulose fiber web with an aqueous solution of synthetic resin in amount and.
having a resin solids concentration such as to be mobile within said web in response to forces established by the drying of said web, and to provide in the web a residual resin solids content of from 20 to 70% by weight of the dried impregnated web, forcing unsaturated air through said web in alternately opposite directions until the volatile content of the web is reduced to within the range of from 20 to 30% of the total weight of the web, and then further drying the web sufliciently to permit handling.
4. The method of producing a substantially uniformly dense cellulose fiber base plastic sheet which comprises impregnating a multi-ply web of creped cellulosic wadding having a dryer basis weight of from 22.5 to pounds per 3000 square feet with an aqueous solution of a phenol-aldehyde resin at a concentration of from 30 to 70% resin solids so as to provide in the web a residual resin solids content of from 20 to 70% by weight of the dried, impregnated web, forcing unsaturated air through said web in alternately opposite directions until the volatile content of the web is reduced to within the range of from 20 to 30% of the total weight of the web, and then further drying the web sufiiciently to permit handling.
References Cited in the file of this patent UNITED STATES PATENTS